1.The Field of the Invention
This invention relates to a method for evaluating the crosslink degree of a vulcanized sample in real time during a vulcanization process or diagnosing the electrical properties of the sample after the completion of the vulcanization process, and then determining an optimal vulcanization time and an optimal content of each constituent gradient of a composition for vulcanization that optimize the properties of the composition according to the vulcanization conditions, thereby improving the properties of the vulcanized sample prepared from a polymer by vulcanization at a high temperature.
2. Related Prior Art
The polymer vulcanization process takes a most time in the manufacture of polymer products in the rubber industry relating to general polymer materials such as tires. Thus steady and persistent efforts have been made to increase the vulcanization rate so as to shorten the required time of the vulcanization process. The conventional methods for reducing the required vulcanization time involve changing the vulcanization system to increase the vulcanization rate or raising the vulcanization temperature. For the method of changing the vulcanization system, a novel crosslink agent of a high vulcanization rate is an essential prerequisite. On the other hand, the method of increasing the vulcanization temperature is effective in reducing the required vulcanization time but changes the crosslinking structure or the crosslink degree depending on the thermal stability of the used polymer or vulcanization system, thus deteriorating the mechanical properties of the polymer composition. For that reason, polymer type, vulcanization system and required properties are the points to be specially considered in reducing the required vulcanization time by raising the vulcanization temperature (See. Elastomer, Vol. 35, No. 3, pp 173-179 (2000)).
The vulcanization reaction is a typical method of forming a three-dimensional reticular structure through a crosslink between main chains of a polymer composition, for example, natural rubber (NR), isoprene rubber (IR), butadiene rubber (BR), styrene butadiene rubber (SBR), butyl rubber (IIR), ethylene propylene rubber (EPDM), chloroprene rubber (CR), chlorosulfonated polyethylene (CSP), nitrile rubber (NBR), acryl rubber, urethane rubber, silicon rubber, fluorine rubber, etc. to enhance the required properties of the polymer composition such as restoring force, elastic properties and the like. There are different types of crosslink formed in the vulcanization reaction. The crosslink widely used in the tire industry is formed by vulcanization introducing polysulfide (—C—Sx-C—, where x is 1 to 4), or by carbon-carbon (C—C) bonding and resinous crosslinking between the main chains of the polymer to increase thermal stability and ageing properties. Recently, the hybrid crosslink that enhances the thermal stability of the polymer is widely used to prevent a deterioration of the properties caused by overcure in the case of crosslinking at a high temperature. The properties of the polymer composition obtained by the vulcanization reaction are greatly dependent on the crosslink density as well as the crosslink type. For instance, the vulcanized crosslinking provides high tensile strength and low thermal ageing characteristic. On the contrary, the carbon-carbon bonding provides high thermal ageing characteristic, low tensile strength and low fatigue fracture. The individual properties appear differently even in a single vulcanization reaction in such a manner that the crosslink density and the modulus of a sample are increased with the progress of vulcanization, but the tensile strength is increased and then decreased with an increase in the crosslink density (See. Polymer (Korea), Vol. 25, No. 1, pp 63-70 (2001)). The vulcanization reaction is a process of adding sulfur or another crosslink agent to the polymer to form a strong and firm crosslink between polymer molecules by pressurizing, heating or other means and to influence the properties of the product, thereby reducing deformation, increasing elastic and tensile strengths and decreasing swelling caused by the solvent used. To enhance the quality and performance of the polymer product with high productivity and reduce the production cost, it is necessary to use a method for effectively increasing the vulcanization rate, a method for preparing an optimal composition for vulcanization, and a method for analyzing the vulcanization process in real time to determine the end point of the vulcanization reaction.
Conventionally, a rheometer method and an impedance measurement method using the scanning of a single frequency have been used to measure the crosslink degree and the crosslink rate of a sample in real time during the vulcanization process in order to improve the properties of the sample prepared from a polymer by vulcanization at high temperature. The rheometer is also called “an oscillating disc rheometer”, which has a rotor causing sinusoidal vibrations three times every minute to deform the rubber and describes a rheometer curve plotting the torque at the rotor axis versus time. The vulcanization rate is measured from the rheometer curve. To calculate the vulcanization rate, a line passing the minimum and maximum values of the torque marked on the curve and parallel with the time axis is first drawn. A second line is drawn to pass the points respectively amounting to 30% and 90% of the line distance and be parallel with the time axis. Then the crosslink time until the second line intersects the vulcanization curve is referred to as t30 and t90, respectively. Subtracting t30 from t90 gives the vulcanization rate. A low value of the calculation means a high vulcanization rate and, in this case, the required vulcanization time of the polymer product is short (See. KP 257,965). In the rheometer method, however, it is necessary to use very expensive rheometer equipment and to keep clean the surface of the oscillating disc being in contact with the vulcanized composition in every vulcanization reactor and the surface of the inner wall of the reactor for the sake of precisely measuring the torque variations of the sample over time. When the shelf time after mixing the composition for vulcanization used for the vulcanization process and all the temperature variations of samples controlled in the vulcanization process are not constant, a considerable error on the vulcanization rate may occur in the rheometer method that measures the macroscopic properties of the sample. It is therefore necessary to establish criteria for evaluation of the vulcanization rate based on the concept of standard deviation. This requires a little over-vulcanization in determining the end point of the vulcanization process, thus increasing the required vulcanization time, and the over-vulcanization inevitably causes a slight deterioration of the properties in the part of the sample. On the other hand, the method for determining the end point of vulcanization by impedance measurement and analysis of the vulcanized sample in real time through the scanning of a single frequency is disclosed by Richard Magill at Signature Control Systems Co. (See. Rubber World, Vol. 221, No. 3, pp 24-28, 62, (1999)). According to this method, the response of the vulcanized sample to a specific frequency (for example, 9 kHz) appears as complex impedance that consists of in-phase (conductance) and out-of-phase (capacitance) components. The conductance component results from the fast movement of ions in the sample and conductance, and the capacitance component is a measure of determining how fast the dipoles in the sample orient to the externally applied single frequency. The orientation speed of dipoles to the frequency applied is generally limited because the hardness of the polymer increases with the progress of the vulcanization process. The reciprocal of the capacitance component measured in real time during the vulcanization process is in accord with the tendency of torque variations observed in the known rheometer method. Thus the measurement of the capacitance component enables the evaluation of the degree of vulcanization and the vulcanization rate. However, the evaluation method using a single high frequency (e.g., 9 kHz) in consideration of fast dynamics in the sample during the vulcanization process is unsatisfactory in accurately predicting all the internal characteristics and properties (including slow dynamics) of the vulcanized sample. The sample during the vulcanization process contains considerably numerous types of ions and dipoles and the internal ion and dipole components are changing at any time with an elapsed time of vulcanization. The mobility of several ions generally shows the slow dynamic characteristic and appears in response to the low frequency. Especially, the behavior of bulky ions can be observed only in the lower frequency range, and the sample evaluation method using the scanning of a single frequency hardly reflects the low dynamics of the sample. Accordingly, the whole wide frequency range (10 kHz to 1 Hz) must be taken into consideration as will be described in the present invention for the sake of the complete analysis and evaluation of a sample in the level of molecule.
As a most basic test in evaluating the properties of a vulcanized sample, the testing method specified in KS M 6518 is applied. First, a thin sample sheet is cut into four dumbbell-shaped test pieces, of which the thickness is then measured with a thickness gauge. Once making 20-mm and 40-mm marks, the test pieces are measured in regard to length and cutting load the moment that they are cut by a tensile tester. The tensile strength and the elongation percentage are calculated according to predefined equations. But the tensile test requires a number of samples and takes a long time for measurement (See. KP 257,965). Besides, the methods for measuring the properties of a vulcanized sample may include exothermic test, tearing energy measurement, repulsive elasticity test, durability test, BFG cutting and chipping characteristic test, all of which still require a long time for measurement of the properties (See. Polymer (Korea), Vol. 25, No. 1, pp 63-70 (2001)).